S 594 

.S5 

Copy 1 ^be XHniversiti? of CbicaGO 



MEASUREMENT OF THE SURFACE 
FORCES IN SOILS 



A DISSERTATION 

SUBMITTED TO THE FACULTY OF THE OGDEN GRADUATE SCHOOL 

OF SCIENCE IN CANDIDACY FOR THE DEGREE 

OF DOCTOR OF PHILOSOPHY 

(department of botany) 



BY 

CHARLES ALBERT SHULL 



A Private Edition 

Distributed By 

The University of Chicago Libraries 



Reprinted from 
The Botanical Gazette, Vol. LXII, No. 
Chicago, 1916 



TLbc TUniversit^ of Cbicaao 



MEASUREMENT OF THE SURFACE 
FORCES IN SOILS 



A DISSERTATION 

SUBMITTED TO THE FACULTY OF THE OGDEN GRADUATE SCHOOL 

OF SCIENCE IN CANDIDACY FOR THE DEGREE 

OF DOCTOR OF PHILOSOPHY 

(department of botany) 



BY 

CHARLES ALBERT SHULL 



A Private Edition 

Distributed By 

The University of Chicago Libraries 

Reprinted from 

The Botanical Gazette, Vol. LXII, No. r 

Chicago, 1916 



sli 

Ob 



SEf '•tt 



VOLUME LXII NUMBER i 



THE 

Botanical Gazette 

JULY 1916 

measurement of the surface forces in soils 

CONTRIBUTIONS FROM THE HULL BOTANICAL LABORATORY 217 

Charles Albert Shull 

(with five figures) 
I. Introduction 

Many investigations of soil moisture have been made, especially 
during recent years, in attempts to learn something of its mechanics 
and its relations to plant growth. In a general way the dominating 
importance of the soil water to plants has long been recognized; 
but notwithstanding the large amount of work done up to the 
present time, we still lack some of the most fundamental, elemen- 
tary facts regarding the physico-chemical relations of the water 
and soil. This is true generally of that region of soil moisture 
which lies between what is called the wilting coefficient of the soil 
and air-dry soil, and more particularly of that critical region 
immediately below the wilting coefficient. 

The discovery of semipermeable coats in seeds in recent years 
by Brown (4), Schroder (30), and others has made it possible to 
measure approximately the force with which the colloidal gels of 
the seed attract water. In a former paper (33) it was shown that 
by means of osmotic solutions whose forces are known the imbibi- 
tion force of a seed at any given moisture content from saturation 
to air-dry could be determined approximately. 

Because of the rapid establishment of moisture equilibrium 
relations after disturbance in Xanthium seeds, this seed has been 



2 BOTANICAL GAZETTE [july 

chosen for a further investigation of the moisture relations of seeds, 
with special reference to the moisture held by soil particles. The 
main purpose of the work was to find some means of measuring 
the force with which particles of soils of varying fineness retain 
moisture at different degrees of dryness, and to obtain some more 
definite knowledge concerning the amount of "back pull" occur- 
ring in soils when the total moisture content is so low as to be 
unavailable to growing plants. Special interest centered in the 
conditions obtaining in the critical region at and just below the 
wilting coefficient. 

This paper presents the principal results obtained during the last 
three years. Since the osmotic method of measuring the internal 
forces of seeds is obviously restricted in practice to such seeds as 
have a perfectly semipermeable coat, a new method was attempted, 
based upon a determination of the vapor pressure equilibrium 
between seeds and osmotic solutions of varying strengths. This 
method has the advantage of being appKcable to all sorts of seeds, 
regardless of the kind of testa present; but since von Schroder 
(31) and Bancroft (2) have shown that colloids may not have 
the same moisture relations to gaseous moisture that they have 
to water itself, the values obtained by the vapor pressure method 
have not been used as the basis of comparison with soils in this 
work. The values for the internal force of seeds as determined by 
osmotic solutions of various strengths will therefore be used as a 
basis for comparing the moisture-holding power of fine soil particles. 

A number of soils have been used in the investigations, and it 
is beheved that the methods of measurement used here will prove 
valuable in many kinds of soil moisture studies, since the deter- 
minations, while giving excellent data as to the physical relations 
of the soil moisture, yield at the same time results of considerable 
physiological significance. The results are more valuable, therefore, 
than purely physical determinations, because they can be inter- 
preted in terms of plant activity. For, after all, it is the plant in 
relation to its environment, not merely the environment, that we 
need to understand. 

The work has been carried on in the Plant Physiological 
Laboratory of the University of Kansas, and in the Hull Botanical 



i9i6] SHULL— SOILS 3 

Laboratory of the University of Chicago, where all needed facilities 
have been generously provided. 

II. Historical 

The general status of our knowledge of the forces operative in 
soils was briefly discussed by Cameron (id) several years ago. 
It is obvious from this account that up to the present time we have 
known very Httle about soil forces within the range of unavailable 
moisture, that is, between the wilting coefficient and air-dry 
condition of the soil. 

The attempts thus far made at measurement of the surface 
forces which are known to exist in finely divided matter of all 
kinds have been made from various angles, but they can be classed 
under two main heads: (a) physical, and {h) physiological. 

PHYSICAL MEASUREMENTS 

A. Heat of wetting method. — ^The principle of heat of wetting 
was discovered by Pouillet (26) a good many years ago. He 
found that all kinds of dry powders, from inorganic substances 
and porous organic matter, yielded heat on being wet with fluids 
like water, oil, alcohol, etc. The organic substances yielded the 
greater amount of heat because, he stated, the organic matter was 
composed of particles incomparably thinner than the finest inorganic 
powders. 

The literature dealing with the appHcation of this principle to 
measurements of surface force has been reviewed so recently by 
Patten (25) that it will not be necessary to go into the details of 
it here. It will be sufficient to point out that through the work of 
Rose (28) and Jungk (19) we gained the conception that water is 
condensed on the surface of the powdered inorganic or finely divided 
organic substances, and that the release of heat is due to this com- 
pression. The quantitative studies of Nageli (23) made it possible 
for Sachs (29) to calculate the surface forces in starch grains. 
Since Joule had shown that 34.3 atmospheres of pressure raises 
the temperature of water 0.03° C, the amount of heat produced 
by starch on being wet would indicate much more than 10,000 
atmospheres of surface force compression. Sachs assumed, of 
course, Nageli's theory of the structure of organic matter. 



4 BOTANICAL GAZETTE [july 

The physicists Young, Dupre, and Lord Rayleigh have esti- 
mated the surface forces of finely divided absolutely dry matter 
at from ii,ooo to 25,000 atmospheres. Lagergren's (20) estimate 
for charcoal fine enough to have 4 sq. m. of internal surface per 
gram was 6150 atmospheres. 

It is probable that this method would give results too high for 
soils, for, as MtJNTZ and Gaudechon (22) have shown, there are 
other sources of heat release than mere compression when absolutely 
dry soil and water are mixed. Heat of solution, dilution, and 
hydration may make considerable errors in estimates of surface 
forces by this means. The statement frequently made that the 
force of surface condensation in soils runs from 6,000 to 25,000 
atmospheres, as by Cameron (10), and by Brown and Smith (5), 
is based upon the discussion previously mentioned. 

B. Compression method. — Rodewald (27) has used a different 
method in measuring the forces on the surface of starch particles, 
which has the advantage of being a direct method; that is, the 
forces of compression are measured by the amount of compression 
produced instead of by the amount of heat produced. He found 
that I gm. of oven-dry starch absorbed 0.326 gm. of water in becom- 
ing saturated. But while the starch swelled, the swelling did not 
amount to as much as the volume of water absorbed. In other 
words, there was a volume loss due to compression of the water. 
The amount of volume loss was 0.0432 cc, and if we refer this to a 
gram of water, the volume loss is equal to 0.1325 cc. per gm. The 
compression coefficient of water is calculated by Wullner to be 
0.00004659 cc. per gm. for each atmosphere of pressure exerted. 
This would give a pressure of 2821 atmospheres for the compres- 
sion actually obtained if we refer the compression solely to the 
water involved. 

By a sKghtly different method of calculating the force of com- 
pression Rodewald obtained a result of 2523 atmospheres, which 
is not referred to the water alone, but to the whole system of starch 
and water. He thinks that the close agreement shows that water 
alone is involved, or that starch happens to have about the same 
coefficient of compressibihty as water. 

The low value obtained by Rodewald as compared with the 
values for inorganic bodies, Patten thinks is due to the fact that 



19 16] SHU LL— SOILS 5 

imbibition and absorption are both involved in the starch, and 
that a much lower value must be obtained than where absorption 
alone occurs. On the other hand, it will give a higher value than 
where imbibition alone occurs. 

While these determinations of the surface force in absolutely 
dry matter are interesting, they have no practical value, for such 
forces as these do not occur in ordinary soils containing capillary 
moisture, or even in air-dry soils and seeds, for it is evident that 
the air-dry soil or seed already holds as hygroscopic moisture the 
water that it would absorb with such remarkable energy if the 
particles were absolutely dry. However, the figures give us an 
idea of the power with which these substances retain the last part 
of their hygroscopic moisture, which must be a force opposite to 
and equal to that with which wetting occurs. 

C. Vapor pressure and centrifugal force methods. — 
Other physical measurements have been worked out, some of which 
are very useful, as for instance Hilgard's hygroscopic coefficient 
(i6), a measure based on vapor pressure relations, and the moisture 
equivalent of Briggs and McLane (6). The latter is particularly 
valuable, since Briggs and Shantz (7) have shown its relation to 
various physical and physiological amounts of water. But only 
one of these measurements can be expressed at present in units 
which permit a comparison of the soil forces with the osmotic 
forces of the roots of plants. 

PHYSIOLOGICAL MEASUREMENTS 

The most important attempt at a physiological measurement 
of the soil forces is that of Briggs and Shantz (8), who use the 
wilting coefficient, or percentage of moisture in the soil at the wilt- 
ing of the plant, in determining unavailable moisture. However, 
recent work by Caldwell (9) and by Shive and Livingston (32) 
shows that within certain ranges the permanent wilting of the 
plant is a function of the intensity of atmospheric evaporation, 
and that the wilting coefficient should be rather a measure of the 
moisture in the plant at the time of wilting than of the moisture in 
the soil. The constancy of this measure is therefore open to some 
question, and its value and Hmitations in physiological studies are 
to be determined. 



6 BOTANICAL GAZETTE [july 

Another important physiological study of soil-moisture relations 
is Alway's (i) investigation of the relation of non-available water 
to the hygroscopic coefficient. He has shown that some kinds of 
plants can remain alive for a considerable time after growth ceases 
from lack of moisture, while others die rather promptly. This is 
doubtless one of the main differences between xerophytes and 
mesophytes. In the case of desert perennial legumes, hfe was 
maintained even after the soil moisture had fallen slightly below 
the hygroscopic coefficient. These results emphasize the need of a 
measure for the surface force of soils which can be expressed, or at 
least interpreted, in biological rather than physical terms. 

There have been few observations on the relation of seeds to 
soil moisture. Bogdanoff (3) studied the relation of germinating 
seeds to soil moisture, and presents many interesting facts. Whit- 
ney and Cameron (36) noted the fact that a quantity of cowpeas 
whose hygroscopic moisture amounted to about 14 per cent, when 
mixed with an equal quantity of soil which contained 15 per cent 
of water, took up 12.1 per cent of their own weight, leaving only 1.3 
per cent of moisture in the soil. That is, the soil was practically 
air-dry. In the paper referred to (33) I have shown that the initial 
internal force of air-dry seeds is little short of 1000 atmospheres; 
if this condition be general among air-dry seeds, the behavior of 
the cowpeas can easily be understood. The relation of seeds to 
soil moisture and vapor pressure will be considered in more detail 
later. 

III. Materials and methods 

Material. — The Xanthium seeds used in the experiments dis- 
cussed in the following section were secured from plants raised on 
the experimental grounds of the University of Kansas in 19 13. 
Originally all of the seeds planted were from a single plant of 
X. pennsyhanicum Wallr. The 119 plants obtained were very uni- 
form in all their obvious characters, and since it has been shown 
(34) that the intermingled local tj'pes of Xanthium are practically 
isolated by differences in the blooming time of each species, the 
seeds may be considered as having come from a fairly pure line. 
This was thought desirable in order that the individual variations 
of the seeds might be reduced to a minimum, and that consequently 



I9I6] 



SHULL— SOILS 



more uniform behavior might be obtained under experimental con- 
ditions. The other seeds used were obtained from local seedsmen 
under the names given. 

The soils used in the major portion of the work will be charac- 
terized briefly. As a representative of heavy clay soil, the subsoil 
of the Oswego silt loam was chosen. Specimens of this subsoil were 
obtained from Riley County, Kansas, on an area about 2 miles 
west of Manhattan. The Oswego silt loam is a residual soil derived 
by weathering from underlying unbedded shales and sandstone, 
with the shales predominating. Its subsoil forms a hard, compact, 
brittle soil, with a gray to dark brown color. The average com- 
position as determined by mechanical analysis is given in table I. 

TABLE I 



Sand 


Silt 




Coarse 


Medium 


Fine 


Very Fine 


Clay 


0.4 per cent 


0.5 per cent 


4.4 per cent 


3.2 per cent 


61.3 per cent 


30.4 per cent 



The moisture equivalent is 35.2 per cent, and the wilting 
coefficient is 19. i per cent. The general details in regard to the 
Oswego silt loam and its subsoil may be obtained from the Eighth 
Report, Field Operations of the Bureau of Soils (11). 

As a contrast to the heavy silt clay, a fine quartz sand, the 
no. 2/0, which is manufactured by the Wausau Quartz Company 
from quartz rock, was chosen. This grade passes through a 124- 
mesh screen, and over a 147-mesh screen. The average diameter 
of the particles is very close to o.io mm. The chemical analysis 
given below shows it to be a very pure quartz sand. 

Silicon dioxide 99- 07 per cent 

Iron oxide 0.17 

Aluminum oxide 0.52 

Hygroscopic moisture o . 06 

Undetermined o. 18 

100.00 

The moisture equivalent is 2.41 per cent and the wilting co- 
efficient 1.3 per cent. 



BOTANICAL GAZETTE 



[JULY 



For a comparative study of the moisture relations of seeds and 
soils at the wilting coefficient of the soil, a series of soil samples 
was obtained from Washington, D.C. The necessary data regard- 
ing these soils are given in table II. 

TABLE II 



Sample 
number 



Locality 



Yuma, Arizona 
Highmore, 

South Dakota 
3 North Platte, 

Nebraska 



North Platte, 

Nebraska 
Amarillo, Texas 
North Platte, Neb 
Akron, Colorado 
Yuma, Arizona 
Akron, Colorado 



Soil type 



Sand 

Loam 
Very fine 

sandy 

loam 

Loam 
Clay loam 
Clay loam 
Fine sand 
Sand 
Loam 



Moisture 
equivalent 



l-35=^004 



23.79^0.10 



15-33= 



= 0.08 



.64 ±0.03 
.65='=o.o2 
. 08 =*= o. 04 
.90=^0.05 
.S3±o.oi 
.91=1=0. 12 



Wilting 
coelBcient 



o. 73=1=0.02 



i2.93=to.o5 



8.33±o.o8 

I2.4I='=0.02 

16. I2=tO.OI 

16.34=^0.02 

3 . 2 1 =fc . 03 

0.83*0.01 

io.82±o.o6 



Hygroscopic 
H,0 (per cent)* 



3 ■'^3 

1.836 

2.28 
3.82 

5-21 

0.7s 
0.218 

2-3 



* The hygroscopic moisture was determined at the time of use in an or dinary dry oven. The other 
figures were furnished by Dr. Lyman J. Briggs, of Washington, D.C. 

Methods. — While the internal forces of Xanthium seeds have 
been approximated by osmotic means, many seeds lack semi- 
permeable coats. For such seeds a vapor pressure method has 
been used which gives results which are in a way comparable to 
the osmotic measurements. It consists essentially in measuring the 
vapor pressure equilibrium of the air-dry seeds over sulphuric acid 
of varying strength, and calculating the internal pressure of the 
seed from the vapor pressure of the solution over which it was 
found to be in equilibrium. Owing to our slight knowledge of the 
concentrated solutions and of the exact relations of colloids to 
water vapor, the calculations can give only a rough estimate of 
the internal forces of the seeds, but they are near enough to the 
osmotic determinations to be of great interest. 

The sulphuric acid series was chosen with some reference to 
the Landolt-Bornstein tables to facilitate calculation. Begin- 
ning with water, the series included 16, 26.5, 35, 39, 50, 54, 57.5, 
66, 73, 84.5, and 96-99 per cent H2SO4. These fluids were placed 
in tightly sealed, small, wide-mouthed bottles. The seeds to be 



19 1 6] SHULL—SOILS 9 

tested were suspended in shallow paper baskets a few millimeters 
above the surface of the acid, the baskets being hung on cotton 
threads fastened to the corks with carna-uba wax. All metalhc 
condensers were thus avoided. After the seeds were carefully- 
weighed and arranged, the bottles were sunk in a trough of running 
water to prevent any considerable changes in temperature. Con- 
densation effects due to change of temperature could not occur 
except over water, for Mitscherlich (21) has shown that even 
10 per cent sulphuric acid will prevent deposition of dew in deter- 
mining hygroscopic coefificients of soils. It may be questioned 
whether the inclosed space actually reaches the vapor pressure of 
the solution, for, as Hilgard (17) points out, it is most difficult 
to secure complete saturation in the case of water vapor. How- 
ever, the space of air to be brought into equilibrium with the 
solution vapor pressure in these experiments is very small, and it 
seems probable that the whole system of liquid, air, and seed 
comes to an equilibrium pressure in the time of the experiment, 
except possibly in the case of water. After allowing 15 days for 
reestablishment of equilibrium by the seeds, the point of no change 
was determined by weighing. 

The osmotic pressure of the sulphuric acid is roughly deter- 

• T !_ n r 1 r 1 ^ /-/' SRT 

mmed by the use 01 the vapor pressure formula P= — 7—. „ , 

in which / is the vapor pressure of pure water at the temperature 
of the experiment, /' the vapor pressure of the acid, M the molec- 
ular weight of the solvent's vapor, T the absolute temperature, 
5 the density of the sulphuric acid, and R the gas constant. The 
osmotic pressure (P) is given in grams per square centimeter, and 
must be reduced to atmospheres. This formula has been developed 
for dilute solutions and does not hold accurately for high concen- 
trations, but there are at present no data on which to base more 
accurate estimations. The boiling-point method yields a result 
close to that given by this formula for sulphuric acid, as will be 
shown later. 

The earliest soil measurements were made with no. 2/0 sand. 
Seeds of known weight were packed firmly in sand of known 
water content in paraffined wire baskets, and allowed to come to 



lo BOTANICAL GAZETTE [july 

equilibrium. The tests were confined finally to the region of soil 
moisture from air-dry to the wilting coefficient, for with a higher 
moisture content the seeds always became saturated with water. 
In the case of this sand it was not until the water content was 
reduced to about i per cent that a noticeable "back pull" was 
developed by the soil. 

This method is obviously open to the criticism that friction 
retards the movement of water in dry soils, and that the seeds 
therefore do not reach actual equilibrium with the total soil mass, 
but only with the soil lying very near them. In order to meet this 
difficulty a rotation method was adopted which brings the seeds 
constantly into contact with fresh soil particles. 

A definite amount of dry soil, usually 60 gm., was taken, and 
the desired amount of water thoroughly mixed with it. In this 
condition the soil was divided finely enough to pass through a 
2 mm. sieve. The moist soil was then placed with dry seeds in 
a wide-mouthed 200 cc. bottle, without completely filhng it, so 
that rotation would constantly mix the soils and seeds. The bot- 
tles were carefully sealed with heavily shellacked corks to prevent 
loss of water during the period of rotation. 

These bottles were arranged on rotating wheels, driven by a 
motor and controlled by a speed reducer (fig. i). The range^from 
air-dry to wilting coefficient was divided into 10 fairly equal 
divisions, giving 11 tests in each series. The rotation was con- 
tinued for 15 days, this time having been chosen after making 
tests as to the effect of dift'erences in duration of the experiment 
on the amount of intake by the seeds. For instance, no. 2/0 sand 
with about 0.2 per cent moisture added permitted an intake of 
22 per cent of their weight by the seeds in 5 days, and a parallel 
test showed 21.7 per cent intake in 10 days. Fifteen days, there- 
fore, seems ample time for the establishment of equilibrium. At 
the end of the time the bottles were opened, and the seeds very 
rapidly separated from the soil and brushed free of all dust with a 
camel's-hair brush. The soil and seeds were both placed in weigh- 
ing bottles as quickly as possible, to prevent serious loss of water 
by evaporation. The soil was weighed carefully and dried at 
104° C. until loss ceased. The seeds also were weighed. The 



I9i6] 



SHU LL— SOILS 



II 



moisture content of the seeds and soils at the time the bottles were 
opened expressed the equilibrium relation of that soil moisture 
content. 

Since means are at hand for determining the internal force of the 
seed at practically any moisture content, it is possible to determine 







j^^ w 


i^dHk ^ 


Bkw^ 


wj^^^Ys 




^ 


mBa 


^^^^^H^^^JmI 


■1 


^P^ 




mg 





Fig. I. — Rotator used in these experiments, with motor and speed reducer. 

from the data the forces of the soils which are in equihbrium with 
those of the seed. 

The principal sources of error lie in the fact that moist soils 
and seeds cannot be handled in ordinary atmospheres without 
some loss by evaporation during the handling, and in the fact that 
hot-air ovens for drying are not as accurate as vacuum driers. 
No claim is made for greater accuracy than these methods will 
permit. Of course, every precaution was taken to reduce errors 
to a minimum, and the work was done with the greatest speed 
and accuracy possible. It is confidently believed that the results 
to be obtained by more refined methods and more expensive 



BOTANICAL GAZETTE 



[JULY 



apparatus would in no way change the nature of the conclusions 
to be drawn from the results. 

IV. Experimental results 

A. Measurement of the seeds.— The measurement of the 
internal forces of Xanthium seeds by means of NaCl and LiCl 
solutions has been repeated and extended with full confirmation 
of the previously published results. The data are presented in 
tabular form for the sake of convenience in table III, and these 
figures may serve as a basis of comparison in the soil experiments, 
where the surface forces of the soil particles, instead of osmotic 
pressure, are pitted against the internal forces of the seed. The 
data were secured with the lower seeds of Xanthium pennsylvanicum. 

TABLE III 

Moisture intake of Xanthium seeds in osmotic solutions; temperature 

23.5° C; INTAKE in percentage OF AIR-DRY WEIGHT 

















Osmotic 


Solutions volume 
molecular 


1 hour 


4 hours 


7 hours 


lo hours 


24 hours 


48 hours 


pressure in 
atmos- 
pheres 


H2O 


16 .^0 


44 38 


48.78 


50 38 


51.18 


51-58 


0.0 


o.iM-NaCl.... 


16 


79 


39-43 


45-87 


46 


48 


46.39 


46.33 


3-8 


o.2M-NaCl.... 


17 


12 


38.67 


45.00 


45 


57 


45-93 


45-52 


7-6 


o.sM-NaCl.... 


16 


07 


34-05 


40.75 


41 


95 


42.24 


42.05 


II. 4 


o.4M-NaCl.... 


14 


36 


31.21 


38.08 


39 


97 


40.33 


40.27 


15-2 


o.sM-NaCl.... 


13 


96 


30.26 


35-87 


38 


08 


38.70 


38.98 


19.0 


o.6M-NaCl.... 


13 


80 


25-57 


32-41 


33 


57 


34.77 


35-18 


22.8 


o.yM-NaCl 


13 


32 


26. 29 


30-99 


31 


73 


32-79 


32-85 


26.6 


o.8M-NaCl 


13 


13 


25-22 


29.21 


29 


95 


31.12 


31.12 


30.4 


o.QM-NaCl 


12 


58 


24-34 


27.64 


28 


95 


29.14 


29.79 


34-2 


i.oM-NaCl 


II 


90 


22.92 


25.42 


26 


48 


26.21 


26.73 


38.0 


2.oM-NaCl.... 


8 


19 


14-55 


18.25 


18 


43 


18.60 


18.55 


72.0 


4.oM-NaCl.... 


4 


81 


8.37 


9,84 


10 


08 


11.00 


11.76 


130.0 


Sat. -NaCl.... 


3 


42 


4-94 


5-24 


5 


84 


6.21 


6.35 


37S-0 


Sat. -LiCl 


— 


67 


-0.77 


-0.58 


— 


-58 


-0.58 


— 0.29 


965.0 



The results obtained by the vapor pressure method over sul- 
phuric acid are shown in table IV, and some of the curves of 
moisture intake showing the point of natural equilibrium are shown 
in figs. 2 and 3. 

As the table and curves show, the seeds are initially in moisture 
equilibrium with sulphuric acid of 46-54 per cent strength. In 
general the seeds which have carbohydrate reserves in greatest 



I9i6] 



SHU LL— SOILS 



13 



abundance seem to have a somewhat lower equiUbrium point than 
those with high fat and protein content. 

The osmotic pressure of the sulphuric acid calculated from the 
vapor pressure formula given runs from 1000 to 1350 atmospheres. 
The vahdity of the vapor pressure formulae will 
be discussed later. If colloids absorbed as much 
moisture from a saturated atmosphere as from 
water, it might be safe to assume that the inter- 
nal force of the seeds is equal to the osmotic 
force of the solution. But if von Schroder's 
work holds for all colloids, this vapor pressure 
method may give abnormal values. If the colloids 
always tended to take up more water when in con- 
tact with the fluid, above the equihbrium point as 
well as below, the values given here would be too 
low, as the equilibrium point would be shifted 
toward the stronger acids. If, on the other 
hand, intake is increased below the equilibrium 
point, and loss is increased correspondingly 
above the equilibrium point, the shape of the 
curve would be changed, but the 
equilibrium point would remain fixed. 



45 



►40 



35 



30 



25 



20 



15 



1-5 



1007. 



845 



73 66 57.554 50 



f > 

39 35 



26-5 



H2O 



Fig. 2. — Curves of moisture equilibrium of seeds suspended over sulphuric acid: 
a, Pisiim sativum; b, Stowell's evergreen sweet corn; c, Xanthium pennsylvanicum; 
abscissae, strength of sulphuric acid; ordinates, moisture intake by seeds in percentage 
of air-dry weight. 

The values run higher with the H2SO4 than with the lithium 
chloride solution, as given in table III. There is one source of 
possible discrepancy which needs to be mentioned. The vapor 
pressure tests were all made in Kansas, where the chmate averages 
drier than at Chicago, while the osmotic measurements were all 



14 



BOTANICAL GAZETTE 



[JULY 



> ^ 

W '^ 

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• • a 

. . <u 


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s 

3 


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3 


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3 


:a ? 


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1 




tiva . . 
tiva. . 
How d 


1 u 

> (U 


anthium 

pennsylvanic 

anthium 

pennsylvanic 

anthium 


"S 

> 


mis. . 
sativu 
tivum 
led see 

R 


3 

13 




in 




ryza sa 
ryza sa 
eid's ye 
corn . - 


Stowell's 
green s 
Feterita 


3? 

c 

c 

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a 


icmus 
commi 
riticum 
sum sa 
(wrink 
)y bean 
upinus 








1 OOCii 


XXX 


P^ H'fL, (^ 


J 1 



I9i6] 



SHU LL— SOILS 



15 



made at Chicago. The hygroscopic moisture of the seeds was 
uniformly near 5.5 per cent in Kansas,' while at Chicago the 
hygroscopic moisture was rarely less than 7 per cent, usually above. 
The equilibrium point would naturally run higher, therefore, in a 
drier climate. However, it is not claimed that the discrepancy 
would entirely disappear if repeated with seeds of 
the same moisture content. 

There seems to be little doubt as to the high 
osmotic strength of the sulphuric acid, for the 
boiling-point method of measuring osmotic press- 
ure gives a value in close agreement with those 
just given. The boiling point of 53 per cent H2SO4 
is 128.5° C. If for each rise of 0.52° C. we may 
assume an osmotic pressure equal to one mole of 
dissolved substance (Hober, Phys. Chem. p. 19), 
this strength of sulphuric acid should have an 
osmotic pressure of 1227 atmospheres. 

At all events, it is safe to con- 
clude that air-dry seeds possess a very 
high internal attraction for water 



a — 




100% 



1.5 



73 66 57.554 50 



39 35 26.5 



16 



H2O 



Fig. 3. — Curves of moisture equilibrium of seeds suspended over sulphuric acid: 
a, Ricinus communis; b, Triticitm sativum; c, Oryza saliva; abscissae, strength of 
sulphuric acid; ordinates, moisture intake by seeds in percentage of air-dry weight. 



which at the initial moment of intake is but little short of 1000 
atmospheres. Owing to uncertainty of the figures for sulphuric 
acid, only those obtained by direct contact with osmotically 
active solutions will be used as a standard for the following soil 
experiments : 

' Seeds sent to Washington were dried in the vacuum oven by Mr. A. B. Camp- 
bell. Upper seeds averaged 5.48 per cent of the absolute dry weight, and lower 
seeds averaged 5. 72 per cent of their absolute dry weight. 



i6 



BOTANICAL GAZETTE 



[JULY 



B. The surface forces of soils. — i. The Oswego silt loam. — 
A number of tests were made with the subsoil of the Oswego silt 
loam, the results of 4 of which are shown in table V. The air-dry 
soil apparently holds its moisture with about the same force as do 
air-dry seeds, a result one might expect, since both are in moisture 
equihbrium with the same atmosphere at air-dry. 



TABLE V 

Relation of soil moisture in subsoil of Oswego silt loam to water intake 
BY SEEDS OF Xanthium pennsylvanicum 



Soil H,0 

in percentage of absolute 

weight 



Intake H,0 
in percentage of 
air-dry weight 



6.66 (air-dry). 

7.8s 

8.92 

10.26 

11-79 

12.74 

14.78 

16.06 

17.3s 

18.07 

19.80 



II 



S . 83 (air-dr>0 . 

6.23 

8.27 

9. 16 

10.81 

13-56 

14-23 

15-34 

17.10 

17-93 

19.71 



0.97 

3-35 
6.25 

958 
11.94 
17.46 
20.62 
34.00 
39-77 
41.79 
46.54 



Soil H,0 

in percentage of absolute 

weight 



Intake H,0 
in percentage of 
air-dry weight 



III 



5.95 (air-dry). 

6.15 

7.61 

8.68 

10.32 

II .60 

13-16 

14.88 

16.75 

18.07 

19 34 



IV 



4.65 (air-dry). 

6.46 

7.88 

9-36 

II. 16 

12.46 

13-91 

15-18 

17.12 

18.87 

20 . 04 



0.38 
1.58 
3-73 
6.16 
10.76 

15-79 
21.36 
28.61 
33-86 
45-15 
49-31 



-0-53 

-hi. 06 

3-68 

6.47 
10.82 
15.81 
21. II 
32.60 
41.98 
47.26 
50.00 



As soil moisture is increased, the moisture intake by the seeds 
increases at a much more rapid rate, until the approaching satura- 
tion of the seed begins to cut down the absorption rate. This 
.happens as the soil moisture approaches the wilting coefficient. 
Reference to fig. 4 will make the relationship of soil moisture 
content to seed intake clear. 



I9I6] 



SHU LL— SOILS 



17 



The general situation is strikingly illustrated by table VI, which 
combines the results of the 4 experiments of table V. While there 
are a number of discrepancies, as one might expect, it is evident 
that the method can be used in measuring approximately the forces 
residing on the surfaces of soil particles at various soil moisture 
contents. For instance, when this soil contains about 9.36 per 




Fig. 4.— Curves showing relation of moisture intake by seeds to increasing soil 
moisture, plotted from experiment 4, table V; abscissae, percentage of soil moisture 
in terms of absolute dry weight; ordinates, percentage of increase of soil moisture 
above air-dry weight for the soil moisture curve (straight), and percentage of intake 
by seeds in terms of their air-dry weight. 

cent of moisture (3.5 per cent above air-dry), it is in moisture 
equilibrium with seeds at 6 . 47 per cent above their air-dry weight. 
The seeds attain about the same equilibrium point with saturated 
NaCl solution, which has an osmotic pressure of 375 atmospheres, 



BOTANICAL GAZETTE 



[JULY 



from which it follows that the surface force of the soil particles 
at the given moisture content is also approximately 375 atmospheres. 

TABLE VI 

Relation of soil moisture to intake by seeds; data of 

TABLE V combined 



Soil moisture in 

percentage of absolute 

weight 

4 . 65 (air-dry) . . . . 

5 . 83 (air-dry) . . . . 

5 . 95 (air-dry) . . . . 

6 . 66 (air-dry) . . . . 

6. IS 

6.23 

6.46 

7.61 

7.85 

7.88 

8.27 

8.68 

8.92 

9. 16 

936 

10. 26 

10.32 

10.81 

II. 16 

1 1 . 60 

"•79 

12.46 

12.74 

1316 

13-91 

14-23 

14-78 

14.88 

15-18 

15-34 

16.06 

16.75 

17. 10 

17.12 

17-35 

17-93 

18.07 

18.07 

18.87 

19-34 

19-71 

19.8 

20 . 04 



Intake by seeds in 

percentage of air-dry 

weight 



-0.53 
0.00 

+0.38 
0.97 
1.58 
1. 91 
1 .06 



73 
35 
68 
,18 
16 
25 
55 
6.47 

9-58 
10.76 

9.81 
10.82 

15-79 
11.94 
15-81 
17.46 
21.36 
21. II 
23-88 
20.62 
28.61 
32.60 

31-54 
34.00 
33-86 
37-70 
41.98 
39-77 
43-25 
41-79 
45-15 
47-26 

49-31 
43-79 
46.54 
50.00 

51-44 



Osmotic pressure equal to surface 
force in atmospheres 



LiCl saturated = 965 atmospheres 



(697)* 
(532) 



(418) 

NaCl saturated = 3 75 



4M.NaCI=i30 
2M.NaCl = 72 



M. NaCl = 38 



M.C6Hi206 = 22.4 



o.5M.NaCl=i9 
o.4M.NaCl = i5.2 



o.3M.NaCl=ii.4 



o.2M.NaCl = 7.6 
o.iM.NaCl = 3.8 



Saturated = o . 00 



* Values in parenthesis calculated from the curve of moisture-holding power of the soil as determined 
by the known values. 



19 1 6] SHU LL— SOILS 1 9 

When the soil moisture reaches 6 per cent above air-dry, the 
moisture intake by seeds indicates a force equivalent to 4M. NaCl 
solution, which is estimated to exceed 130 atmospheres. At 11 
per cent above air-dry the holding power of the soil has fallen to 
22.5 atmospheres approximately. 

In this manner, comparing the percentage intake from the soil 
with that from the solutions, as given in table III, one may estimate 
the surface force for any given moisture content of the soil, each 
soil type, of course, having specific relations. If it were possible 
to make absolutely accurate determinations for several points in 
the curve of moisture intake by seeds, as related to the curve of 
moisture increase in a particular soil, it would be a simple mathe- 
matical problem to calculate the exact water-holding power of 
the soil particles at any soil moisture content whatsoever for 
that soil. 

2. The no. 2/0 sand. — By preliminary tests ranging from 17.5 
per cent to i per cent of moisture it was found that there was no 
significant water-holding power in this sand until the moisture 
content fell to less than 2 per cent. At i per cent of soil moisture 
the seeds took in over 45 per cent of their own dry weight. 

The results of a series of tests running from air-dry (o. 14 per 
cent) to a little beyond the wilting coefficient (1.3 per cent) are 
shown in table VII. 

TABLE VII 
Relation of moisture in no. 2/0 quartz sand 

TO MOISTURE INTAKE OF Xauthium SEEDS 



Soil H,0 

in percentage of absolute 

weight 


Intake H,0 

in percentage of air-dry 

weight 


0.14 (air-dry) 

0.159 

0.17s 

0.203 

0.44 

0.81 


-0.306 

+ 1.407 

5.02 

21.81 

33 98 

42.40 

45.64 
47.46 
52.06 
72.85* 


I 03 

1 . 40 


1.70 


2. 14 





*Four seeds showing incipient germination, hypocotyls 
averaging 3 mm. long. 



20 



BOTANICAL GAZETTE 



[JULY 



In this sand the very rapid decrease in the force with which 
its particles hold water as moisture content increases stands in 
sharp contrast to the slower decrease in the heavy clay subsoil. 
While the air-dry sand gives the same kind of result as the air-dry 
clay, by the time the sand contains 0.44 per cent of water the 
seeds secure as much water as they do in a molecular solution of 
non-ionizable salts. This indicates a force of 22.4 atmospheres. 

3. Various soil types. — The results obtained with the subsoil of 
the Oswego silt loam and the no. 2/0 sand suggested that there 
might be a general relationship between soils and seeds as regards 
the amount of moisture seeds will absorb at the wilting coefficient 
of the soil, whatever value the wilting coefficient might have. To 
clear up this point, the soil types of table II were used. Each soil 
was brought as nearly to the wilting coefficient as possible by 
addition of water. The closeness of the experimental conditions 
to the wilting- coefficient determinations is shown in columns 3 and 4 
of table VIII. 

TABLE VIII 

Relation of wilting coefficient to moisture intake by seeds 



Soil types 


Percentage of 

hygroscopic 

moisture 


Percentage of 
wilting coefficient 


Percentage of 
soU H>0 


Percentage of 
seed intake 


1. Sand (coarse) 

2. Loam 

3. Sandy loam (very fine) 

4. Loam 


0.205 

3 130 
1.836 
2.280 
3.820 
5.210 
0.750 
0.218 
2.30 


0.73=^0.02 
12.93=4=0.05 

8.33=1=0.08 
12.41=1=0.02 
l6.I2=tO.OI 

16.34=^0.02 
3.21=1=0.03 
0.83=1=0.01 

10.82=1=0.06 


0.65 

12.66 

7.86 

13-30 
16.01 

17.78 

3-19 

0.80 

10.51 


34 
49 
48 

49 
49 
47 
49 
40 

50 


44 
02 

38 


5. Clay loam 


49 
31 
77 
98 
42 


6. Clay loam 


7. Fine sand 


8. Sand (coarse) 

9. Loam 





The percentage of moisture taken up by dry seeds placed in 
each soil is shown in the last column of the table. With the excep- 
tion of the two sands from Yuma, Arizona, which are coarse, the 
results are fairly uniform. In two cases the soils adhered badly 
to the seeds, making accurate work very difficult; but corrections 
were made as carefully as possible. In all the other soils the seeds 
remained clean, or were easily brushed free of adhering particles. 
The average intake for the 7 types, excluding the coarse sands, is 



i9i6] SHULL— SOILS 21 

approximately 49 per cent, and this average agrees rather closely 
with the intake in the Oswego silt loam subsoil and in the no. 2/0 
sand at their wilting coefficients. This probably means that the 
wilting coefficient represents a fairly de&iite water-holding power 
for the soil particle, regardless of its size. By comparison of the 
results given in table VIII with those given in tables III and VI, 
it is seen that the "back pull" of the soil particles, the force with 
which they withhold water from seeds and plants at this critical 
moisture content, is not more than the equivalent of o. iM. NaCl 
solution, or 3-4 atmospheres. This value is surprisingly low. 

V. Discussion 

In this paper the term internal forces of seeds is used to designate 
the resultant of all forces within the seed tending to cause intake 
of moisture ; and by surface forces of soils is meant the resultant of 
all the forces of adhesion, cohesion, surface force, etc., as determined 
by size, chemical composition, and character of the surface of the 
soil particles, which cause the soil to retain moisture. 

In order to make clear the nature of the problems involved in 
this work we shall first consider rather fully the moisture relations 
between seeds and their environment, and then the moisture rela- 
tions of soils to seeds and to the root hairs of living plants. A 
careful study of the moisture relations existing between organic 
bodies, like seeds, and the atmosphere, the soil, and osmotic solu- 
tions, if the seeds have semipermeable coats, convinces one that 
the entrance or exit of water from the seed is due to the interplay 
of such internal forces as capillarity, imbibition force, colloid 
hydration forces, etc., with external forces such as atmospheric 
evaporation, the surface forces active on soil particles, osmotic 
pressure, etc., according to the environment of the seed. Moisture 
flows into or out of the seed as one or the other of these sets of 
forces is the greater. Movement of water continues only until 
the two forces, unequal at the start, become equal. This establishes 
moisture equilibrium, and further movement of water must be 
consequent to some disturbance, external or internal, of the bal- 
anced condition of forces. Moisture equilibrium may obtain at 
any moisture content of the seed, if only the two forces are equal. 



22 BOTANICAL GAZETTE [july 

The attempt to measure the internal forces of seeds with semi- 
permeable coats by means of osmotic solutions was based upon 
this conception of the moisture relations, and on the assumption 
that the total osmotic pressure of the solution is transmitted through 
the membrane as force when pitted against the internal forces of 
the seed through the agency of the semipermeable coat. The 
results of this attempt have been highly satisfactory, and I can 
see no good reason for doubting that the internal forces of air-dry 
seeds are approximately equal to 900-1000 atmospheres. 

The vapor pressure method, using sulphuric acid, involves 
another assumption whose validity may be a little more question- 
able. It is generally admitted that the osmotic pressure of a solu- 
tion can be calculated with some degree of accuracy from its vapor 
pressure. I have assumed in addition that the vapor pressure of 
the seed hydrogels measured against the vapor pressure of strong 
solutions can be used as a measure of the internal forces of the seed. 
The assumption seems reasonable enough, but it would be difficult 
to offer definite proof of its validity at present. 

The principal difficulty with the vapor pressure method is the 
uncertainty as to the osmotic pressure of sulphuric acid. It may 
seem quite unwarranted to some even to think of estimating the 
osmotic pressure of strong sulphuric acid by means of a formula, 
knowing as little as we do of all the factors which enter into the 
problem in this case, and knowing that these formulae have all 
been developed for the dilute "ideal" solutions. I realize fully 
the danger of basing any conclusions upon results obtained by such 
precarious methods, and offer the results merely for their suggestive 
value. The formula I have used for calculating the osmotic pres- 
sure from the vapor pressure is that given by Walker (35), with 5 
representing the density of the solution rather than that of the 
solvent. This is necessary in order to make the formula fit a 
concentrated solution with high density. This same formula has 
been used by others for the same purpose, as by Drabble and 
Drabble (12). 
•Attention is called to a difference in the formulae given by 

Nernst (24) and Walker. In Walker's discussion, --— repre- 



i9i6] SHULL— SOILS 23 

sents relative lowering of the vapor pressure of the solution as com- 
pared with that of water. His formula actually gives the relative 

P — P^ 
lowering. Nernst, however, uses for this factor in the 

equation, and with concentrated solutions it is quite a different 
thing. In a concentrated solution, where the relative lowering of 
the vapor pressure is 0.75, Nernst's formula will give a value for 
the osmotic pressure 4 times as great as that given by Walker's 
formula. 

In the third English edition of Nernst he gives an equation 
for this calculation which he claims gives a very exact value for 
the osmotic pressure from vapor pressure. His equation is as 
follows : 

„ 0.0821. r. 10005 , p 

" = ^r:i^ • ^« 

M p' 

The physical chemists claim that this formula should hold in so 
far as it includes the factors involved. But even this exact 
formula does not take care of the change in volume occurring on 
dilution of the acid, nor for the heat of dilution, which is very 
considerable in the strong acid solutions over which equilibrium 
of moisture vapor was obtained in these experiments. 

I have chosen to use Walker's formula because it actually 
gives the relative lowering of the vapor pressure, as it is supposed 
to do. However, it is perfectly clear from this discussion that the 
osmotic pressure of sulphuric acid of any given concentration, 
especially of high concentration, cannot be measured accurately 
by any single vapor pressure formula suggested to date. 

The measurements made over sulphuric acid give values, there- 
fore, which are merely suggestive. They indicate that the internal 
forces of air-dry seeds of all kinds are very high, and confirm the 
high values obtained by the osmotic solution method. But the lat- 
ter method only, I beheve, can be relied upon for the measuring 
of the internal forces of seeds until the vapor pressure relations are 
more perfectly understood. 

The main conclusion in regard to these seed measurements may 
be stated briefly thus: It is possible, with seeds having a perfectly 
semipermeable coat, to measure the water-attracting internal forces 



24 BOTANICAL GAZETTE fjULY 

residing in the seed substance at any moisture content between 
saturation and air-dry. The actual values for this range in Xan- 
thium seeds are given in the last column of table III. 

The results obtained by measuring the soils with measured seeds 
are of the greatest interest. While it is important to have means 
of determining in terms of atmospheric pressure the water-retaining 
power of soils at any degree of dryness between saturation and air- 
dry, it is still more important to understand the moisture relations 
of the plant to the soil at and just below the wilting coefficient. 
Does wilting occur because capillary soil forces and osmotic root 
forces reach equilibrium ? If so, why should Briggs and Shantz 
(8) have found a uniform wilting coefficient for all kinds of plants 
when we know that root cells vary somewhat in osmotic concen- 
tration from species to species ? And why should this uniformity 
fail in the presence of intense evaporation, as shown by Caldwell 
(9), and by Shive and Livlngston (32) ? These questions, and 
the discrepancies between the excellent work done at Washington 
and at Tucson can probably be answered intelligently, and explained 
in the light of these experiments. 

The moisture equivalent of a soil is the percentage of water left 
in the soil after centrifuging for a certain time under a force of 1000 
gravities, a force about equal to one atmosphere. We have been 
accustomed to find the wilting coefficient empirically, or to divide 
the moisture equivalent by 1.84. But we have not known how 
much greater are the soil forces at the wilting coefficient than at 
the moisture equivalent. From the results obtained, the pressure 
value of the wilting coefficient seems to be about 4 atmospheres. 

As the soil becomes drier and drier, the forces become greater 
and greater, until on the approach of air-dry conditions a very 
small change in moisture content makes a very large change in the 
forces involved. The increase of the force with decreasing soil 
moisture is shown graphically in fig. 5, which shows the curve of 
increasing force for the Oswego silt loam subsoil, and for no. 2/0 
sand. The difference in the curves for clay and sand is striking. 
In the clay, where the surface films are relatively thick, the force 
decreases very slowly for a considerable distance. But as the films 
become very thin, the surface force and forces of adhesion of water 



igi6] SHULL— SOILS 25 

to particles become very great. This force would reach its maxi- 
mum presumably when the particles were surrounded by a film of 
water just one molecule thick. The curve for the sand is very simi- 
lar, except that the period of slow increase of the soil forces is very 
short, and that the whole curve lies much nearer 
to the absolutely dry condition. These rela- f-IOOO 

tions, of course, are conditioned mainly by the 
size of the particles. 

Let us see now how the soil forces at the 
wilting coefficient compare in value with the 
average osmotic pressure of the root hairs. A 
few years ago Hill (18) showed that the root 
hairs of Salicornia can in a measure accommo- 
date themselves to changes in the osmotic con- 
centration of the surroundings, through increase 
or decrease of the cell sap concentration parallel 
to the changes in the environment. That the 
cell sap of leaves and other exposed parts in- 
creases in concentration with xerophytic habitat 
has been shown by Drabble and Lake (13), 
Fitting (14), and others. The general conclu- 
sion reached by Drabble and Drabble (12), 
that the osmotic strength of cell sap varies 
with the physiological scarcity of water in any 
area, seems most reasonable, and it doubtless 



^ 



' ^00 
ZOO 

■700 
'600 
■500 
► ^00 

300 

100 

100 




807. 60 40 20 

Fig. 5. — Curves showing increase in the surface forces of soils as drying proceeds; 
to the left, for subsoil of the Oswego silt loam; to the right, for no. 2/0 sand. 

holds true of root cells under xerophytic soil conditions. As 
the soil becomes deficient in water supply, and the surface forces 
are rapidly increasing in magnitude, we might then expect this 
increased force of the soil to be paralleled by increase in the osmotic 
forces of the root hairs, owing to progressive concentration of the 
cell sap. Such increase might amount to many atmospheres. 



26 BOTANICAL GAZETTE [jui.y 

The most careful, extensive, and valuable study of the osmotic 
pressure of the sap of root cells under ordinary conditions of soil 
moisture which has been made up to the present is that by Hannig 
(15). He finds that the average root cell sap pressure for 64 species 
of plants is o. 21M. KNO3, or equivalent to 7 or 8 atmospheres. It 
is seen, therefore, that the water-holding power of the soil at the 
wilting coefficient is only about half that of the average osmotic 
pressure of the sap of the root cells. Certainly wilting at the wilt- 
ing coefficient cannot be due to lack of water, for seeds come within 
a few per cent of taking up as much moisture at the wilting co- 
efficient as when placed in water itself. Nor can it be due to 
equalization of forces between root hair and. soil water, for there 
is still a gradient of 4 atmospheres of force in favor of the plant. 
Moisture and gradient for movement of water toward the plant 
are both present, and yet the plant wilts ! 

Even in cases where the soils are drier than the wilting coefficient, 
the accommodation of the root hairs mentioned above would prob- 
ably maintain this gradient of a few atmospheres in favor of mois- 
ture intake. This idea is strongly supported by unpublished work 
of Miss Edith A. Roberts, who, working in this laboratory, has 
shown that seedlings of mustard and radish grown in sugar solu- 
tions develop root hairs with osmotic pressures usually about 4 
atmospheres in excess of the medium in which they grow, the same 
amount of excess as this gradient at the wilting coefficient. This 
relationship of internal to external forces was maintained, in her 
work, up to volume molecular solutions of cane sugar. It is exceed- 
ingly probable, therefore, that as soils dry out beyond the wilting 
coefficient the root hairs maintain an osmotic pressure a few 
atmospheres in excess of the soil forces until those forces become 
relatively very high. Nevertheless, permanent wilting occurs 
within a narrow range of soil moisture under moderate conditions 
of evaporation. 

There seems to be but one reasonable explanation for this situ- 
ation. The wilting of plants at the wilting coefficient of the soil must 
be due to the failure of water movement from soil particle to soil 
particle, and from these to the root hairs, rather than from lack 
of moisture or gradient. This does not mean complete cessation 



19 1 6] SHU LL— SOILS 27 

of movement of film water toward the plant. It is a question of 
rates. Evaporation continues from the leaves in accordance with 
atmospheric conditions somewhat regardless of conditions below 
the soil surface. At the wilting coefficient the film water becomes 
so stable, and the friction of movement becomes so great, that the 
rate of movement of water toward the root is quite inadequate to 
meet the needs of the plant, and permanent wilting ensues. 

It becomes clear at once why Briggs and Shantz, working 
under rather uniform conditions of evaporation, found the same 
wilting coefiicient for all kinds of plants in a given soil, regardless 
of variability of root sap concentration and other variable factors, 
for these variables do not aft"ect the point at which capillary move- 
ment of water over the soil particles ceases to be effective for the 
plant. This is determined by the physical properties of the soil, 
the fineness of the particles being the chief factor. We should 
expect, therefore, this uniform behavior under moderate conditions. 
On the other hand, when the evaporation rate is very intense, the 
plant might be caused to wilt permanently before this wilting 
coefiicient is reached, owing to the fact that after all it is a question 
of rates. The rate of movement in the soil fails to be adequate 
sooner. 

In concluding this discussion, may I suggest that the methods 
used and the conclusions reached in this work should receive very 
critical consideration by plant physiologists, soil physicists, and all 
others interested in these problems. We have lacked even the 
most elementary facts concerning these important moisture rela- 
tions of the soil. This is a first attempt to throw light upon an 
unexplored region of soil physics. It is hoped that other methods 
may be devised for testing the correctness of the conclusions 
reached by the methods presented here. The apparatus is quite 
simple and easily used. If the results obtained can be fairly 
substantiated by other methods, the method will be exceedingly 
valuable in physiological and ecological investigations of many 

kinds. 

VI. Summary 

I . The force with which the seeds of Xanthium pennsylvanicum 
absorb water has been measured by two methods: (a) osmotic 



28 BOTANICAL GAZETTE [july 

solutions, and {b) vapor pressure equilibrium. The osmotic method 
is at present the more reliable. 

2. The air-dry seeds of Xanthiiim show an initial attraction for 
water of nearly looo atmospheres. 

3. The attraction which exists at any moisture content of the 
seed between air-dry and saturation can be approximated. Table 
III gives the data. 

4. The seeds have in turn been used to measure the complex 
moisture-holding forces of soils, with the following results: 

a) The air-dry subsoil of the Oswego silt loam holds its hygro- 
scopic moisture with about the same force as an air-dry seed, that 
is, about 1000 atmospheres. 

h) As the moisture content of the soil increases, the surface 
force decreases rapidly. When about 3 . 5 per cent of water has 
been added to the air-dry soil, the force remaining is about 375 
atmospheres. When the soil moisture reaches 6 per cent above 
air-dry in this soil, the moisture is held with a force of 130 or more 
atmospheres. At 1 1 per cent above air-dry the holding power has 
fallen to 22.4 atmospheres. 

c) At the wilting coefficient of the soil (13.3 per cent above 
air-dry in the Oswego silt loam subsoil) the "back pull" of the soil 
particles amounts to not more than that of a o. iM. NaCl solution, 
that is, not more than about 4 atmospheres. This is shown to hold 
true for a number of t>pes of soil with widely varying wilting 
coefficients. 

5. This water-holding power of soils at the wilting coefficient 
is less than the osmotic pressure of the root hairs of many kinds of 
plants, as shown by Hannig and others. 

6. The wilting of plants at the wilting coefficient of the soil 
cannot be due to lack of moisture in the soil, nor to lack of a gradient 
of forces tending to move water toward the plant. 

7. The view is held, therefore, that the wilting at this critical 
soil moisture content must be due to the increasing slowness of 
water movement from soil particle to soil particle, and from these 
to the root hairs, the rate of movement falling below that necessary 
to maintain turgidity of the cells of the aerial parts, even under 
conditions of low transpiration. 



19 16] SHU LL— SOILS 



29 



My thanks are due to Professor L. E. Call, of the Kansas State 
Agricultural College, Manhattan, for sending me the subsoil of the 
Oswego silt loam used in these experiments, and to Dr. Lyman J. 
Briggs and his assistants in the biophysical laboratory of the 
Bureau of Plant Industry for making determinations of the moisture 
equivalent of the local soils, hygroscopic determinations of the seeds 
in the vacuum driers, and for the various soil types which were so 
kindly sent to me. I desire also to express my appreciation of the 
generous way in which the Hull Botanical Laboratory has provided 
all needed facilities for the work, and especially my indebtedness 
and gratitude to Dr. William Crocker for much helpful advice 
and encouragement received during the progress of the work. 

University of Kansas 
Lawrence, Kan. 



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30 BOTANICAL GAZETTE [july 

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